Identification And Characterization Of Resistance To Broomrape Of Sunflower

-Reference number:AIR-CT-93-6111
-Name of trainer:Serenella A. Sukno

-Summary of achievements and main conclusions.

We identify sources of resistance to highly virulent populations of Orobanche cernua, in both, from cultivated and from wild species of Helianthus. Immunity or a very high level of resistance to this population was observed. We characterize and transfer new sources of resistance to highly virulent races of broomrape from wild Helianthus species to cultivated sunflower .

The re-evaluations of our collection of wild Helianthus using a new highly virulent population of O.cernua confirm a high frequency of resistant entries among perennial wild species accessions in comparison to the annuals. In a previous work (Ruso et al., 1994) it was found that all the perennial entries tested were immune to two populations of broomrape. In the present study two accessions H. gracilentus and H. nuttallii ssp. nuttallii showed segregation for resistance. The results of evaluating annual species confirm the resistance of H. exilis and H. anomalus previously reported (Ruso et al.,1994). Moreover, another species, H. agrestis, not tested before was found to be resistant. The high levels of resistance found in many of the accessions, especially perennials, suggest the usefulness of wild sunflower species as sources of genes for resistance to O. cernua. The higher cross compatibility of the annuals with cultivated sunflower suggests these species are preferable in breeding programs. The resistance of the F1 plants between H. exilis and susceptible lines of cultivated sunflower indicate that the resistance to O. cernua, present in this species, is dominant. The segregation observed in one of the crosses suggests that all the resistance genes are not in homozygous condition in all the individuals of H. exilis. Unfortunately, the limited results of this study, do not allow us to interpret the segregation rate (19:3) found in the progeny of H. exilis 1 x P21. In order to elucidate it, backcrosses with the susceptible line are under testing. The results of the BC1 evaluation will determine the mode of inheritance of broomrape resistance found in this species.

The resistance found in H. exilis, could provide new genes of resistance against populations of O. cernua occurring in Spain as well as to new races overcoming current sources of resistance. The interspecific hybrids evaluated in our study indicate that it is possible to use this species to incorporate the resistance in H. exilis into cultivated sunflower. We conclude that it is possible to transfer broomrape resistance genes from wild annual species into cultivated lines in breeding programs for resistance to this disease. Interspecific hybridization has become important as a means of introducing resistant genes into the cultivated sunflower. This has been facilitated in wild perennials species by the use of embryo culture and chromosome doubling with colchicine to increase the fertility. We have obtained 9 interspecific hybrids with wild perennial sunflower including hybrids of the diploid species: H.gracilentus , H. nuttallii, H. giganteus, of the tetraploid H. hirsutus, and of the hexaploid species H. californicus, H. pauciflorus , H. resinosus. And the two supposed tetraploid that had been found hexaploid H. decapetalus and H. laevigatus, and the cultivated lines HA89 and P21. Four of them are resistant to broomrape and different generations of backcross are under testing. The other five interspecific hybrids, recently obtained, will be tested together with both parents (resistant and cultivated) in Spring of 1996. Our results like others suggest that seed set is higher when the wild specie is used as the pollen parent than when used as maternal parents. In this work we used the wild species as female. Although the cross is more difficult in this direction, the wild cytoplasm with its diversity is conserved. The introgression into cultivated sunflower of these genes of resistance could reduce genetic vulnerability to this disease and provide more genetic diversity to the narrow genetic base in cultivated material. For the long term future, introduction of boomrape resistance genes from perennial Helianthus species will hopefully alleviate the threat of the newly evolved virulent broomrape race.

Six cultivated sunflower (H. annuus L.) sources from different origins resistant to O. cernua Loefl., one carrying the gene of resistance Or5 and two susceptible lines, along with the F1 crosses (resistant x susceptible and resistant x resistant) and segregating generations (F2 and BC1F1) were studied to characterize the gene number and mode of inheritance of resistance to broomrape. Segregation ratios of F2 and backcross generations indicate one major dominant gene controlling resistance. The lack of segregation in the F2 and BC1 F1 generation between resistant lines indicate that the gene for resistance in the different resistant lines is allelic to or closely linked to Or5 gene. Further evaluations of all the resistant lines of this study to other new more virulent inoculum, which overcome resistance given by Or5 gene were carry out. Two of the resistant lines used in this study, JB-2 and Kavk showed resistance to the new population (SE296). These results suggest that these lines may have a allele or gene differentfrom the Or5 whichwas not possible to distinguish with inoculum SE194. This resistance to the new virulent broomrape population SE296, is an important finding since resistance of current sunflower cultivars is based on Or5 gene.

The results of segregating generations of an interspecific hybrid of a wild annual sunflower resistant species, H.exilis and a cultivated susceptible line suggested that two genes could be involved in the resistance of this species and that they are different from Or5. This was an interesting possibility since the resistance of Or5 was starting to be overcome by new virulent races.

Interspecific hybrids and backcross generations betweenthe resistant wild perennial species Helianthus resinosus Small, H. pauciflorus Nutt., H. laevigatus T.&G., H. nuttalliinuttallii T.&G.and H. giganteus L., H. hirsutus, H. californicus and H. gracilentus and the broomrape susceptible H. annuus L. cultivated inbred line HA89 were made to study the behavior of hybrids and backcrosses on the transmission and expression of resistance to this parasite. The resistance of the F1 plants between the wild perennial speciesand HA89 indicated that the resistance is dominant in this species, thus facilitating its transfer in backcross programs. In the case of the diploid H. giganteus and H. nuttallii, resistant plants with 34 chromosomes, the diploid number of cultivated sunflower, were obtained whereas in the case of BC1F1 of hexaploid perennials x H. annuus, the resistant plants had 51 chromosomes. Segregation for susceptible and resistant individuals indicated that the transfer of resistance found in these species with different ploidy levels into cultivated sunflower was feasible.

Soybean mosaic virus (SMV) and peanut mottle virus (PMV) cause yield loss and seed quality deterioration including seedcoat discoloration. Reports of the relationship between seedcoat mottling and virus infection are inconsistent. Research was conducted to examine the response of soybean genotypes when inoculated with SMV or PMV and to study the association of virus infection with the degree of seedcoat mottling. The effect of inoculation with PMV at 2 growth stages was also studied. Preliminary results of this research indicated a strong association between symptoms expression of virus infection and seed coat mottling with both virus. SMV infection produced more severe disease symptoms and greater reductions in plant height, but lower percentages of mottled seeds and less severity of mottling than did PMV infection. Early PMV infection produced higher percentages of mottled seeds and greater reductions in seed size than did late infection. A few genotypes showed no leaf symptoms of virus infection but produced some mottled seeds. This observation needs further investigation.